The present invention relates to an electrophotographic printing device of the type having an exposure unit comprising the combination of a plurality of light-emitting diodes which are light sources and a plurality of means for transmitting the light beams emitted from the light-emitting diodes and converging or focusing them like converging lenses (to be referred to as "the light-beam transmission-and-convergence means" in this specification). The last mentioned optical means may be, for instance, the products of Nippon Sheet Glass Co., Ltd. sold under the trademark of "SELFOC". This optical means is described in "Optical Characteristics of a Light-Focusing Fiber Guide and Its Applications", Uchida, et al, IEEE Journal of Quantum Electronics, Volume QE-6, No. 10, Page 606, Oct. 1970.
The prior art electrophotographic printing devices of the type having an exposure unit comprising a cathode-ray tube and an optical fiber plate (a plurality of optical fibers are arranged in a plate form) have been well known. The electron beam emitted from the cathode-ray tube is transmitted through the optical fiber plate so as to be focused on the surface of a photosensitive member. However, the optical fiber plate only has a function of transmitting the light beam therethrough and one end of the optical fiber plate is desirable to be made into contact with the surface of the photosensitive member. Furthermore, the optical fiber plate must be moved relative to the surface of the photosensitive member, so that the optical fiber plate must be spaced apart from the surface of the photosensitive member in order to avoid damage to the surface of the photosensitive member by friction. Thus, the prior art exposure unit must satisfy the above two requirements which are contradictory to each other. As a result, with the prior art exposure unit, a distance of from 50 to 100 .mu.m must be maintained with a high degree of accuracy between the optical fiber plate and the surface of the photosensitive member. Therefore, the prior art exposure unit needs a mechanism for supporting the optical fiber plate which is complex in construction and which must support the optical fiber plate with a high degree of accuracy. As a result, the prior art exposure unit is very expensive. In addition, toner tends to adhere to the end surface of the optical fiber plate, so that exposures are adversely affected and subsequently the electrostatic latent images are degraded or deformed. Moreover, the use of a cathode-ray tube results in an increase in size of the exposure unit.
In order to make the exposure units compact in size, there has been proposed the use of light-emitting diodes. With the light-emitting diodes, the exposure units can be made 1/10 in size as compared with the exposure units using a cathode-ray tube. However, as with the cathode-ray tube exposure units, the distance between the end of the optical fiber plate and the surface of a photosensitive member must be strictly maintained. Thus, the light-emitting diode type exposure units still need a complex and expensive mechanism for supporting the optical fiber plate.